The qualitative characteristics of the immune responses are regulated by T cell subsets through their production of distinctive cytokines. Well-characterised T cell subsets are T helper (Th) cells, of which two different subsets are recognised. Th1 cells, which through the production of IFN-γ, promote cell-mediated responses, and Th2 cells which, by producing IL-4, IL-5, IL-13, induce antibody synthesis, as well as mast cell and eosinephil responses. Both subsets originate from a naive T cell precursor, whose differentiation is influenced by both the modes of activation and the environment. A number of variables known to influence the development of T cell subsets includes the interaction of the T cell receptor (TCR) with antigen, the presence of antigen presenting cells (APC) and the presence of certain cytokines during activation of the T cells.
A third subset of T cells, so-called regulatory CD4+ cells or regulatory T cells, is described that predominantly produces interleukin-10 (IL-10)1. IL-10 inhibits a broad array of immune parameters, including activation and effector function of T cells, monocytes and dendritic cells (DCs), limiting and ultimately terminating inflammatory responses2, 3. These IL-10 producing T cells are named regulatory T cells type 1 (Tr1), because of their immunosuppressive effects both in vitro4 and in experimental colitis1. Tr1 cells produce little IL-2, no IL-4 and the production rate of IL-5, IFN-γ and transforming growth factor beta (TGF-β) is similar to those of naïve T cell clones. The mechanisms by which regulatory T cells mediate immunosuppressive activities in vivo are still unknown, but some regulatory T cell populations are known to be dependent on IL-10 for their function5, 6.
Studies of IL-10 deficient (IL-10−/−) and IL-10 receptor-2 deficient mice7, 8, which show that these mice develop a T helper (Th)-1 mediated intestinal inflammation in the absence of IL-10 or IL-10 mediated signalling indicate that IL-10 is an important regulatory cytokine within the mucosal immune system. The activity of IL-10 in counter regulating mucosal inflammation is likely to be multifactorial. IL-10 is a potent down regulator of IL-12 production and thus acts at the level of Th1 cell induction.9 In addition, IL-10 suppresses production of other proinflammatory cytokines and chemokines including TNF-α, IL-1, IL-6 and IL-8.3 Finally, there is substantial evidence that IL-10 acts both to promote the differentiation and augment the activity of regulatory T cells.1, 6, 10, 11 
The observations in IL-10−/− mice have laid the foundation for therapeutic trials of IL-10 in several other models of colitis. These studies have shown that systemic IL-10 administration is able to prevent intestinal inflammation by down-regulating the intestinal proinflammatory Th1 response.7, 12, 13 Based on these successful experimental findings, recombinant (r)IL-10 was administered by subcutaneous injection to patients with either mild/moderate or steroid refractory Crohn's disease, as well as in patients undergoing ileal resection to prevent postoperative recurrence.14-16 Although the data indicated that systemic rIL-10 therapy is safe and well tolerated, this therapy did not result in significantly higher remission rates or clinical improvement compared to placebo. Explanations for this lack of efficacy include the short half-life of rIL-10,17 local delivery of insufficient amounts of rIL-10 to inhibit mucosal Th1 responses and the side effects associated with high dose rIL-10.18 
Sustained IL-10 delivery may prove more effective than daily systemic injections.
These limitations of systemic rIL-10 therapy might be overcome by the infusion of in vitro generated regulatory T cells to patients with T cell-mediated inflammatory diseases. However, the low proliferation rate of regulatory T cells and the high cost of rIL-10, required for their generation in vitro, seriously hamper the production in vitro of therapeutically effective quantities of regulatory T cells.
In an experiment to overcome the anti-proliferative properties of IL-10, antigen-specific murine T cells transduced with IL-10 have been employed (Setoguchi et al., 2000, J. Immunol. 165: 5980-5986). Splenic T cells from mice transgenic for an ovalbumin-specific TCR were transduced with a retroviral plasmid containing the murine IL-10 gene. The IL-10-transduced ovalbumin-specific T cells were subsequently infused into mice with antigen (ovalbumin) induced arthritis. The transduced T cells migrated to the inflamed joint and ameliorated the arthritis symptoms of the joint. However, this use of IL-10 transduced T cells is strictly limited to T cells that are specific for a predetermined antigen. As for most (if not all) inflammatory diseases the relevant antigens are not known, this antigen-specific application of IL-10-expressing T cells currently has no practical therapeutic value.
In experimental autoimmune encephalomyelitis (EAE), a Th1 mediated disease, efficient delivery to the site of inflammation of either therapeutic19 or exacerbating factors20 by genetically modified T cells has been reported. Encouraging results were obtained from a study showing that antigen specific T-cell clones transfected with IL-10 cDNA were able to inhibit EAE19 However, because of the antigen specificity of the transfected T cell clones these results again have no practical therapeutic value.
Thus, it is an object of the invention to provide for IL-10 producing regulatory T cells for use in the therapy of inflammatory diseases in an antigen-independent manner, as well as to provide for methods of obtaining such regulatory T cells. In general, it is an object of the invention to provide for mononuclear cell populations that are derived from peripheral blood mononuclear cells (PBMCs) and that are transgenic for IL-10. Such IL-10 transgenic mononuclear cell populations or specific subfractions thereof may be used as therapeutic agents in a variety of inflammatory diseases, in particular in Th1-mediated inflammatory diseases.